Stomatal pores control gas exchange including H2O loss (transpiration) and CO2 uptake. Understanding the regulation of stomatal pores is important for the future genetic improvement of gas exchange. The hormone abscisic acid (ABA) causes stomatal pores to close under dehydrated conditions. Although much of the ABA signaling pathway is unknown, it has been proposed that an increase in cytoplasmic Ca2+ is the earliest known ABA signaling event We have developed a novel method to load the calcium indicator, Calcium green-I, into the cytoplasm of guard cells as well as epidermal cells of Commelina Co mm Un is, which has enabled us to monitor the spatio-temporal changes in Ca2+ concentration in response to ABA by using laser fluorescence confocal imaging techniques. Furthermore, we have extended this approach and are pursuing molecular genetic studies to determine how ABA-induced Ca2+ increases are effected in ABA signaling mutants of Arabidopsis thaliana. We are using the confocal microscope and image analysis facilities at the NCMIR, including the program B track developed at the NCMIR, to monitor calcium transients. These studies are allowing a cellular analysis of the function of cloned genetic loci for ABA signaling in Arabidopsis.